1. Field of the Invention
This invention relates to catheter imaging and more particularly to catheter imaging using a rotary encoder to determine rotational position and change.
2. Previous Art
Catheter imaging devices have earned an established place in the treatment of diseases of the vessels of the circulatory system. In treating such diseases, the physician inserts the catheter imaging device into a vessel or other biological conduit to produce an image of a portion of the interior of the vessel. The interior vessel image is typically reproduced on a computer-type monitor screen. The displayed image assists the physician in determining the existence and the extent of disease and in selecting and carrying out an appropriate course of treatment. The more accurate and detailed the image, the greater the assistance rendered.
Recent advances in the ultrasonic imaging art now hold out the hope of very high resolution medical imaging. Ultrasonic imaging transducers are becoming available which are capable of better than 100 micron resolution. Such high resolution imaging makes possible highly detailed images of the interior of coronary arteries. Such images are of great value to the physician. Yet this advanced technology is currently limited because of an inability to obtain accurate rotational information with respect to the orientation of the transducer. Without such accurate information, attempts to use these advanced transducers produce images which are often fuzzy, out of focus or worse, merely unrecognizable streaks.
A catheter imaging device is disclosed in copending U.S. continuation-in-part application Ser. No. 08/091,160 by Milo et al., filed Jul. 13, 1993, hereby incorporated by reference. The disclosed device includes a cutter housing attached to the distal end of a torquable catheter body. A circular cutting blade and an ultrasonic imaging transducer are disposed within the housing and are secured to the distal end of a rotatable torque cable. An elongated aperture formed along one side of the housing allows the intrusion of diseased tissue which may then be severed by rotating and axially translating the cutting blade. An electrical connection is made back along the torque cable between the ultrasonic transducer and electronic circuits used to operate the transducer for imaging.
The catheter imaging device is inserted into a coronary artery whose interior is to be inspected. The torque cable is rotated causing the imaging transducer to rotate within the cutter housing. The electronic circuits drive the rotating ultrasonic transducer causing it to emit short bursts of ultrasonic energy through the aperture in the side of the housing. The emitted energy is reflected from the walls of the surrounding vessel, passing back through the aperture to the rotating transducer. There the reflected ultrasonic energy is converted into a low level electrical signal. This signal is processed by the electronic circuits. The processed results are used to display an image of the portion of the interior of the biological conduit adjacent to the lateral opening. This is the image used by the physician.
The rotation of the imaging transducer produces a sweeping motion of the radiated ultrasonic energy across a portion of the interior of the vessel. The results of this sweeping motion permit the electronic circuits to create a two dimensional displayed image. Such an image is particularly helpful to the physician in visualizing a diseased condition and in the guiding and the controlling of the cutting blade.
In order to create an accurate two dimensional image, the electronic circuits need precise information relating to the rotational orientation and rotational speed of the ultrasonic transducer with respect to the aperture, which serves as a frame of reference. Effective use of the new imaging transducers, with resolutions of better than 100 microns, place a premium upon the accuracy of the available rotational information.
In many existing devices such rotational information is obtained from a rotary encoder placed near the proximal or driven end of the torque cable. A rotary encoder is a device which is used to determine the rotational orientation and/or speed of a shaft with respect to a stationary reference. An optical encoder is a rotary encoder which is read optically, such as by fiber optics.
Experience with catheter imaging devices using the proximal end encoder placement has shown this configuration to be satisfactory when used with lower resolution transducers. But when used with the newest high resolution transducers, proximal encoding is susceptible to severe image distortion such as the fuzziness, lack of focus and unrecognizable streaks mentioned above. The cause of the distortion appears to be related to slight differences in rotational speed between the two ends of the torque cable. Under this theory, the distortion results because the electronic circuits are being provided rotational information which differs from the actual rotation of the ultrasonic transducer.
One possible explanation for this phenomenon is as follows. Over brief time intervals, the proximal and distal ends of the torque cable rotate at different speeds. While the proximal end of the torque cable rotates at the driven speed, the distal end will sometimes rotate more slowly and at other times more rapidly than the driven end. The phenomenon is referred to as "non-uniform rotational distortion."
The amount of this distortion is dependent in part upon the materials and the type of construction used in making the torque cable. This part of the distortion is believed to be the result of the torsional "springiness" of the flexible torque cable.
An additional source of the rotational distortion is believed to be caused by resistance encountered by the torque cable as it passes through the catheter. The catheter typically follows a twisting, circuitous route as it threads its way through the circulatory system. Sometimes the torque cable will bind against the enclosing catheter at a sharp bend. As this happens, the torque cable will be slowed momentarily, storing torsional energy along its spring-like length. Then the cable will release suddenly and for a brief interval will rotate at a higher speed as the spring unwinds. This part of the distortion tends to be unpredictable and is difficult to characterize. Thus, the electronic circuits cannot compensate for these effects.
Over a longer period of time these uncertainties average out and the two ends of the drive cable make the same number of revolutions. But for brief intervals, the rotational speed differences between the proximal and distal ends of the drive cable can be enough to distort the displayed image of the interior of the biological conduit by providing inaccurate rotational information. The resulting images may be useless to assist the physician at his task. Thus the advantages presented by the availability of high resolution imaging transducers are lost.
What is needed is a way to accurately measure the rotation of the ultrasonic transducer. This must be accomplished in some manner which is not plagued by the effects of non-uniform rotational distortion within the torque cable.